Normally, and this is normally, you'll find boarding beginning around noon - give or take 30 to 45 minutes. It's usually most crowded for the first hour or so. And as I said, they want you at the pier 90 minutes prior to sailing, so the latest you'll want to get there is 2:30. And keep in mind that on many ships now they don't allow you to get into your cabin until around 1:30. So if you arrive before then, you have to hang around on the Lido Deck, which is quite crowded. This is why we normally arrive around 1:30 - 2:00, as there are usually no lines and when we check in, we can go right to our cabin.
Just some food for thought.